The increasing use of ion sensors in the field of clinical, environmental, agricultural, industrial and medicinal analysis is putting pressure on analytical chemists to develop new sensors for the fast, accurate, low-cost, reproducible and selective determination of various ion species containing nitrogen (N), phosphorus (P), potassium (K), or sulfur (S) continuously across a wide dynamic range of several orders of magnitude, i.e. from micro-molar to molar levels.
For example, due to the vital importance of phosphate in many biological, environmental and industrial systems, there is a need for phosphate-selective ion sensors for the monitoring of mono-hydrogenophosphate (HPO4) or di-hydrogenophosphate (H2PO4) in aqueous solutions. The phosphate anion plays an important role in vivo. For example, in signal transmission systems, a variety of information transmissions can be controlled via the phosphate functional groups of phosphorylated proteins or phospholipids. It is therefore expected that an established sensing system for detecting phosphate anions in an aqueous solution corresponding to an in vivo environment will serve as a basic tool in cell biology and other fields for the analysis of a number of in vivo processes, the results thereof contributing to the development of new medicines and reagents. For example, the recognition of an intracellular phosphorylation signal, a key reaction for the malignant alteration caused by an abnormal information transmission, will be effective in designing inhibitors and the like against such reaction.
Existing ion sensors based on optical or potentiometric monitoring of an analyte often lack the necessary dynamic range required for the accurate and continuous determination of ion species concentration.
U.S. Pat. No. 7,864,321 (CARON) provides an optical fiber sensor wherein a chemical indicator is provided in the cladding, causing a variation of the optical absorption as a function of ion concentration in the solution. Such an optical method may suffer from intrinsic signal-to-noise limitations in the optical source/detector system, constraining the sensor to a detection dynamic range of less than three (3) orders of magnitude, which is not adapted to an accurate and continuous determination of ion species from micro-molar to molar levels. Furthermore, the broadband spectroscopic apparatus described in U.S. Pat. No. 7,864,321 tends to be very expensive.
Ganjali et al. in Analytica Chimica Acta 481 (2003) 85-90 provides for potentiometric monitoring of phosphate in aqueous solutions. Although a change of six (6) orders of magnitude in concentration of phosphate can be measured continuously, the monitoring scheme relates to a change of chemical potential in the electrolyte of only about one (1) order of magnitude. This small variation of transduction yields a large error of measurements of concentration, up to 50% measurement error, which for most applications is not adequate to the accurate determination of ion concentration in a solution.
U.S. Pat. No. 5,296,123 (REDDY) provides an electrochemical sensor adapted for use in the electrochemical analysis of liquids. The sensor performs alternating current (AC) and direct current (DC) voltammetry measurements in plating bath solutions in order to measure the electrical current through the liquid which is indicative of the quality of the electroplating solution. Although sub-milliampere electrochemical measurement methods may be adapted to provide accurate and continuous impedance readings through a large dynamic range, the sensor described in U.S. Pat. No. 5,296,123 does not provide for a selective determination of ion species containing nitrogen (N), phosphorus (P), potassium (K), or sulfur (S).
U.S. Pat. No. 7,521,250 (HAMASHI) provides a fluorescent sensor for phosphate ion and phosphorylated peptide, comprising a phosphate anion-selective fluorescent compound fluorescing at an optical wavelength of 380 nm. Although fluorescent methods provide selective monitoring schemes for ions, the intrinsic signal-to-noise limitations in optical detector systems at 380 nm constrain the sensor to a dynamic range of typically less than three (3) orders of magnitude, which is not adapted to an accurate and continuous determination of ion species from micro-molar to molar levels. Furthermore, ultraviolet (UV) optical systems capable of reliably detecting light at an optical wavelength of 380 nm tend to be very expensive.
Thus, known sensors and methods lack the full set of attributes needed for obtaining ion sensors for the fast, accurate, low-cost, reproducible and selective determination of various ion species containing nitrogen (N), phosphorus (P), potassium (K), or sulfur (S) with a wide dynamic range of several orders of magnitude, i.e. from micro-molar to molar levels. There therefore exists a need in the art for an improved electrochemical ion sensor which alleviates at least some of the drawbacks of the prior art.